Optimizing Sparsely Patterned Polymer Brushes for Guiding Block Copolymers to Form Large-Scale Ordered Lines

Abstract

Directed self-assembly (DSA), which combines the self-assembly of block copolymers with conventional photolithography, can improve lithographic precision by achieving density multiplication, and is thus regarded as one of the promising next-generation lithography techniques. One of the most applicable patterns fabricated via DSA is the high-density vertical stripe pattern, and its core challenge lies in controlling the defect concentration to an extremely low level. In principle, a properly designed guiding pattern can direct symmetric AB diblock copolymers to form thermodynamically stable stripes without any defects. However, guiding patterns generally have multiple critical parameters that are difficult to synergistically optimize in experiments, which lowers the thermodynamic stability of perfect stripe structures and accordingly increases the probability of generating defects. In this work, large-scale simulations are performed using coarse-grained molecular dynamics simulations to investigate the self-assembly of AB diblock copolymers guided by patterned templates consisting of periodically grafted homopolymer brushes and random copolymer brushes, focusing on the influences of the chemical affinity of homopolymer brushes, the length of polymer brushes, and the composition of copolymer brushes on the directing effect. Taking two-fold density multiplication (i.e., DM = 2) as an example, we find that individually increasing the chemical affinity or the brush length can broaden the compositional window of the copolymer brushes for forming defect-free stripes, yet the broadening effect is limited. Simultaneous optimization of these two key parameters can widen the compositional window to a much larger degree. It is revealed at the molecular scale that excessively strong affinity and overly long polymer brushes disrupt the formation of vertical stripes. Furthermore, a typical set of optimized parameters obtained for DM = 2 is directly applied to the case of DM = 3, and defect-free vertical stripes are successfully obtained in a number of independent simulations within a large area of 21L₀ × 21L₀ (L₀ denotes the stripe period).